Photoinactivation of Salmonella enterica (serovar Typhimurium) by tetra-cationic porphyrins containing peripheral [Ru(bpy)2Cl]+ units

Abstract In the last years, antimicrobial photodynamic therapy (aPDT) has been considered as one important alternative to be used in antimicrobial treatments. In the present work we report the first photoinactivation of Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) by cationic free-base meso-tetra-(phenyl)porphyrin and its Zn(II) complex containing peripheral Ru(II)-bipyridyl complexes. Bacterial inoculum (107 CFU mL-1) was incubated with the porphyrins (25 μM) with an irradiation system containing a halogen lamp with fluence values of 0, 180, 360, 720 and 1080 J cm-2. Both porphyrins demonstrate phototoxicity against bacteria, but higher efficiency was observed by Zn(II) complex. It was demonstrated that RuTPyP and ZnRuTPyP phototoxicity is due to their photodynamic activity associated with the formation of singlet oxygen, anion superoxide and hydroxyl radical at their irradiation by visible light. The formation of these reactive species was confirmed using flash-photolysis technique and spin-trap electron paramagnetic resonance (EPR) spectroscopy. The singlet oxygen quantum yield was determined by indirect method using the uric acid as singlet oxygen scavenger. Besides the characterization of optical absorption and fluorescence RuTPyP and ZnRuTPyP properties were realized using optical absorption and fluorescence spectroscopies. It was observed that complexation with Zn(II) ion increases the quantum yields of porphyrin fluorescence and singlet oxygen formation.